1. Field of the Invention
The present invention relates to an image forming method utilizing an electrophotographic process.
2. Related Background Art
An electrophotographic system is widely employed in an image forming apparatus for example a copying apparatus, a printer or a facsimile for forming an image on a recording medium such as paper. In the electrophotographic system, an image bearing member such as a photosensitive member is uniformly charged on a surface thereof, which is irradiated with a laser light to generate a potential difference between an irradiated portion and a non-irradiated portion thereby forming an electrostatic latent image. Then a charged toner is deposited on the surface of the image bearing member, thereby developing the electrostatic latent image on the surface of the image bearing member as a toner image. Thereafter, the toner image is transferred onto a recording medium to form an image thereon.
As charging means for charging the surface of the image bearing member, a corona discharge apparatus or a contact charging apparatus is utilized. The corona discharge apparatus is effective for charging the surface of the image bearing member at a specified potential, but involves drawbacks of requiring a high voltage source and generating ozone. On the other hand, the contact charging apparatus charges the surface of the image bearing member at a specified potential by contacting a voltage-applied conductive charging member with the surface of the image bearing member, and has features of not requiring a high voltage source, a much smaller ozone generation in comparison with the corona discharge apparatus, and a simpler structure.
As the toner image is formed repeatedly on the surface of the image bearing member, it is necessary to sufficiently remove, after the toner image transfer to the recording medium, a residual toner which is not transferred thereto but remains on the surface of the image bearing member. As a cleaning member in such electrophotographic system, there is employed a cleaning blade, which is a counter blade constituted of an elastic material. A method of eliminating the residual toner by contacting such cleaning blade with the surface of the image bearing member is widely utilized because of a low cost, a simple and compact configuration in the entire electrophotographic system and an excellent toner eliminating efficiency. Such cleaning blade is generally constituted of urethane rubber, which has a high hardness and a high elasticity and is satisfactory in an abrasion resistance, a mechanical strength, an oil resistance and an ozone resistance.
Physical properties of the cleaning blade and a mode of contact thereof with the image bearing member are significantly affected by an ease of cleaning depending on a level of adhesion of the transfer residual toner to the image bearing member and a surface property of the image bearing member. Also since the cleaning property is significantly influenced by physical properties of the toner such as a shape, a particle size and a material thereof, it is necessary to select a blade matching the toner and to set an angle and a contact load appropriate for the image bearing member.
A higher image quality and a lower running cost are recent requirements for the image forming apparatus. In the photosensitive member employed as the image bearing member in the electrophotographic system, a thinner photosensitive layer is adopted for achieving a higher image quality, and, for a lower running cost, improvements are being made on an electrical strength, a mechanical strength and an abrasion resistance of the surface of the photosensitive member, in order to extend the service life of the photosensitive member.
However, an image forming apparatus employing such image bearing member is found to have the following drawbacks.
In an image bearing member of a high durability having a high strength and a high abrasion resistance, particularly in an image bearing member of a very high abrasion resistance showing a surface abrasion of 2 mg or less in a Taber abrasion tester, the image bearing member is hardly refreshed by a surface scraping and tends to accumulate, over a prolonged period, an electrical damage by charging, a surface deterioration by a deposition of discharge products, and a mechanical damage caused by a friction with the cleaning blade. Also a sliding property of the surface of the image bearing member (particularly that to the cleaning blade) is lowered to cause a vibration, a squeaking and a tuck-up of the cleaning blade. Also as the surface of the image bearing member is not easily scraped, the discharge products are not easily removable thereby leading to an image deletion. Therefore, various measures have been proposed for solving this drawback.
As an example, Japanese Utility Model Publication No. H01-34205 proposes a method of heating the image bearing member with a heater, thereby avoiding a low electrical resistance on the surface of the image bearing member caused by a moisture adsorption and preventing image smearing. However, such heater, also requiring thermal control means, complicates the configuration of the image forming apparatus, also leading to a complication in the system against the trend of the copying machine and the printer toward a compacter and more personal system. Also such heater requires a certain temperature elevating time, involving a long warm-up time from the start of power supply to the actual printing operation, and also involving an electric power for this purpose. Also the heating of the image bearing member, close to a glass transition temperature (Tg) of the toner, may cause sticking of the toner onto the surface of the image bearing member.
Japanese Patent Application Laid-open No. S61-100780 discloses another method to eliminate the discharge products by rubbing the surface of the image bearing member with an elastic roller. This method can provide a sufficient rubbing force, but the transfer residual toner, eliminated from the surface of the image bearing member and sticking to the elastic roller, is carried thereon unless eliminated by other means from the elastic roller, and is repeatedly subjected to a rubbing between the image bearing member and the elastic roller, thus causing a fused adhesion. On the other hand, even when the elimination from the surface of the elastic roller is achieved sufficiently, in case the transfer residual toner is present only in a small amount, the elastic roller comes into direct contact and rubbing with the surface of the image bearing member, thus damaging the surface thereof.
Also, Japanese Patent Application Laid-open No. S61-278861 proposes still another method to use a developer containing an abrasive, thereby actively eliminating the discharge products. This method, which does not require a new member such as a heater or an elastic roller in the aforementioned rubbing method, allows to simplify the apparatus and to reduce the cost thereof.
However, in case an aforementioned contact charging apparatus is employed as the charging means, the abrasive leaking through the cleaning blade may contaminate the contact charging apparatus, thereby inducing an uneven charging to cause an image defect.